Electroluminescent (hereinafter, “EL”) displays are dominantly emissive flat panel displays created for example by interposing a layer of luminescent material between two insulator layers and two conductor layers to which a controllable voltage can be applied, creating a controllable electric field over at least a portion of the luminescent material for excitation, and thus making it luminous at the location of excitation. At least the one of said conductors is at least partially transparent to allow the luminescent radiation (usually visible light) to leave the display structure for viewing purposes. Usually said layers are thin, their thickness is in the order of some 10s to 100s of nanometers (nm). Such displays are thus called Thin Film EL displays, (“TFEL” displays or “TFELs” for short).
When a voltage is applied to said conductors, the layer of EL material (“luminescent material”) emits radiation in some emission wavelengths, giving rise to an emission spectrum. This spectrum can consist of one or more continuous wavelength regions where said emissions take place, separated by regions with no or only negligible emissions. For display purposes, said emission spectrum comprises at least one band of wavelengths of visible light. Said conductors are usually arranged to form a matrix of row and column electrodes, giving rise to the picture elements or pixels of the display device. Such a display is thus called a “matrix display”. It is also possible to arrange the electrodes into segments of arbitrary symbols or shapes. In this case the segments can be lit independently of one another. Such a display is called a “segmented display”. A TFEL display also features control electronics which is connected to the electrodes of the TFEL display. Control electronics is usually not visible to the viewer of the display, and outside the image forming display area of the TFEL display.
The light emission color of TFEL displays depends on the physical properties of the material used as a luminescent layer of the active layer. This layer is also called the “phosphor” layer in the display community. Typical materials are e.g. ZnS:Mn (zinc sulphide doped with manganese) and ZnS:Tb (zinc sulphide doped with terbium) for yellow and green emission colors, respectively. The emission ranges of these substances are rather narrow, covering only a part of the visible wavelength spectrum. A typical performance level of a commercial state-of-the-art TFEL display is a luminance of 100 nits (=100 candelas/m2) or more.
Thin film electroluminescent (TFEL) technology is well known, and many important aspects of TFEL technology like the basic physics, typical materials, electrical operations, driving methods, long term stability and reliability issues and manufacturing technology of TFEL displays are common and general knowledge. In particular, AC-driven, inorganic thin film electroluminescent (TFEL) displays have reached a mature stage. In such displays, the display is driven with an alternating current (and voltage) with both substantially positive and negative driving voltage signals. Further, in such displays, luminescent layers, insulator layers and conductor layers of the display are substantially of inorganic material. A good overview of related technology is available from the book “Electroluminescent Displays” by Yoshimasa A. Ono, World Scientific Publishing Co., 1995 (ISBN 981-02-1920-0), in particular from Chapters 3, 5 and 8.
Visible light is the portion of the electromagnetic (hereinafter, “EM”) spectrum to which the human eye is sensitive, causing the sense of sight or vision. The spectrum of the visible light (“visible spectrum”) has a wavelength of approximately 380 nm-760 nm. Human eye interprets different wavelengths of visible spectrum of light as different colors. For example, light with wavelength of 580 nm is seen as yellow, light with wavelength of 545 nm is seen as green, and light with wavelength of 690 nm is seen as red color. In case there are many wavelengths present, the sense of vision interprets the aggregate radiation according to the well-known color theory. White light is a suitable combination of light components of different wavelengths (e.g. three components: red, green and blue).
Primary properties of any EM radiation, including visible light, are intensity, propagation direction and speed, frequency or wavelength spectrum, and polarisation. Propagation speed of the EM radiation in vacuum c0=299,792,458 m/s is a fundamental constant. However, for any non-vacuum media, speed of the EM radiation is lower. Light speed in media with refractive index n is simply c0/n (speed of EM radiation in vacuum divided by refractive index of the media). Intensity of radiation (also known as power density) is the power transferred per unit area (W/m2) by the EM radiation. For any viewing application, the intensity of visible light must be sufficient for the sense of sight to detect information. To convey information, information conveying light must also be detectable from the background ambient light by the sense of sight, meaning that the contrast of the information conveying light must be high enough relative to the ambient light.
An important, emerging subtype of thin film electroluminescent displays is the transparent thin film electroluminescent display, denoted also as “TASEL” or “TASEL display” for brevity. These displays are usually inorganic, AC-driven displays, but other types such as DC-driven displays or organic light emitting displays (OLED) are also possible. Transparent TFEL displays possess the significant advantage of allowing the viewer (or user) of the display to access simultaneously both the information shown on the display and information or events which are present or take place behind the display. Vehicle dashboard meters (e.g. tachometer and speedometer), neonatal intensive care unit displays and display cabinets for luxury goods with digital signage are examples of applications where it is very advantageous to see behind the display device and through the display device so that virtually nothing is obstructed from the view and maximal information is conveyed also from behind the display to the viewer. Additional information on prior art transparent TFEL technology is set forth for example in a public white paper by Abileah et al., “Transparent Electroluminescent (EL) Displays”, published by Planar Systems, Inc. (2008).
Inorganic, thin film electroluminescent (TFEL) technology is especially well suitable for transparent display applications as it provides a light emitting display with potentially very high transparency with photopic transmission (as defined later) values of being higher than 50%. Consequently, unless otherwise indicated, the word “transparent” in the present application means a structure that passes light in the visible spectrum so that the photopic transmission of the structure, and in particular, the TASEL, is above 30%, more preferably above 40% and most preferably above 50%.
The main difference between transparent and conventional TFEL displays is that the opaque metal electrode material (typically aluminium) is replaced by transparent electrode material (typically indium tin oxide, ITO) so that the electrodes (and naturally, other possible layers) on both sides of the luminescent layer are suitably transparent to light. Irrespective of the display type, all TFEL displays are known for excellent picture quality, rugged design and long-term reliability.
A significant drawback of the prior art transparent TFEL displays, TASELs, is their performance in bright ambient light. Naturally, the intensity of the light emitted from the EL pixels or segments must be such that the light conveying the displayed information is clearly observable in the ambient lighting conditions by the sense of sight (also called “eyesight”). The ambient lighting conditions are unfortunately more difficult to control with TASELs than with more traditional, non-transparent TFEL displays where the completely opaque backside of the display device blocks a large portion of the ambient light.
An important measure of the display performance in ambient light is the maximum achievable contrast ratio. To make the information conveyed by the display as easy to observe as possible by the sense of sight, the contrast ratio should be as high as possible. A simple way to estimate the contrast ratio CR is given by the following equation:CR=(LEM+LAM)/LAM,where
LEM=Luminance of a display pixel or display segment when pixel/segment is active (in a luminance state), and
LAM=Luminance of a pixel or segment originating from ambient light (pixel or segment is in a non-luminance state).
From the equation above it is easy to see that anything that decreases LAM and increases LEM would improve the contrast ratio and consequently improve display's ability to convey information for the viewer.
Prior art approaches for increasing contrast ratio CR include the idea of simply increasing LEM, and in practical terms this can be achieved e.g. by driving the display with more power. However, there is an upper limit to the power imposed by physical characteristics of the display device. Further, power consumption must usually be minimized in any electrical appliance, especially if the appliance is portable and operated mostly or solely under battery power. As already discussed, decreasing ambient light with prior art methods in transparent displays has been challenging especially in outdoor conditions where, during daytime, sunshine creates a very strong ambient luminance from the display (ambient luminance means the light intensity reflected from or passed through the display as perceived by human eye falling upon the display viewing side surface side or backside surface from ambient light sources such as Sun, indoor lighting or car headlights).
For a transparent display, another important property is the transmission of light through the display, best characterized by a photopic transmission coefficient T, of the display over the whole visible light range as perceived by human eye, originating from a standard light source.
As EM radiation (EM radiation is also called EM waves) interacts with the media differently at different frequencies (and correspondingly, wavelengths), TR (subscript R for spectral radiometric transmission) is wavelength (“λ”) dependent (TR=TR(λ)). TR is a spectral radiometric quantity, and it indicates the ratio of the power (or a related quantity, energy) of transmitted EM wave and incident wave at some material interface or interfaces at a certain wavelength. A TASEL or other such transparent optical device is naturally one such relatively complex example of such surfaces and material interfaces. TR can be measured using a double beam spectrometer (one beam measuring the incident, the other the transmitted wave) which produces a transmission spectrum between some wavelengths λ1 and Δ2.
To get a more realistic transmission information related to human vision, TR(λ) must be weighted with a photopic spectral response of a human eye V(λ), as only wavelengths contributing to the sense of sight are relevant in display applications—power carried by the radiation outside the visible spectrum is, from the standpoint of vision, lost.
As radiation sources also exhibit a frequency (and thus, wavelength) dependent response, to further increase the accuracy of the transmission analysis related to human vision, spectrum characteristics I(λ) of a light source should be taken into account, too. For example, it is possible to express I(λ) as a standard CIE-D65 light source, denoted as I(λ)D65, a commonly used standard illuminant defined by the International Commission on Illumination (CIE) that corresponds roughly to a midday sun in Western Europe or Northem Europe.
The combined result of V(λ), I(λ) and TR(λ) is known as the visible light transmission or photopic transmission TP, measuring the brightness of an object (e.g. display) radiating according to a standard spectral response I(λ)D65 as perceived by a human eye, having response V(λ):
      T    p    =                                          ∫            0            ∞                    ⁢                                                    I                ⁡                                  (                  λ                  )                                                            D                ⁢                                                                  ⁢                65                                      ⁢                                          T                R                            ⁡                              (                λ                )                                      ⁢                          V              ⁡                              (                λ                )                                      ⁢            d            ⁢                                                  ⁢            λ                          ⁢                                                        ∫          0          ∞                ⁢                                            I              ⁡                              (                λ                )                                                    D              ⁢                                                          ⁢              65                                ⁢                      V            ⁡                          (              λ              )                                ⁢          d          ⁢                                          ⁢          λ                      .  
A basic requirement for transparent displays like TASELs is that the value for photopic transmission TP of the display structure is high, more than 30%, more preferably more than 40% and most preferably more than 50%, as otherwise the transparent nature of the display starts to suffer. With careful design, value of 65% or even higher for the photopic transmission TP of a TASEL is achievable. However, as already discussed, prior art transparent displays suffer from the high intensity of ambient light that the high photopic transmission of the transparent display allows to pass through, cutting down the contrast ratio, and seriously hampering the readability and usability of the transparent (e.g. TASEL) displays.
It is also possible to express the transmission of an optical element like light source or a reflector without taking into account the physiological characteristics of vision, and to take into account the special emissive characteristics of the light source. For example, for the characteristic emissivity of EL displays, I(λ)EL, the electroluminescent transmission TEL can be defined as
      T    EL    =                              ∫          0          ∞                ⁢                                            I              ⁡                              (                λ                )                                      EL                    ⁢                                    T              R                        ⁡                          (              λ              )                                ⁢          d          ⁢                                          ⁢          λ                    ⁢                                          ∫        0        ∞            ⁢                                    I            ⁡                          (              λ              )                                EL                ⁢        d        ⁢                                  ⁢        λ            
A related quantity is the electroluminescent reflectance, REL, defined here as REL=1−TEL−L. This means that amount of light emitted by the EL light source (such as an EL display) not transmitted through the surface of the light source is reflected back at the surface, or lost into the optical loss mechanisms (expressed as coefficient L) of the source.
By studying the expression for TEL it is evident that in the numerator there is a product of two factors, I(λ)EL and TR(λ). If either one of these factors is zero or close to zero, the product is zero or close to zero, and the contribution to the integral is also zero or close to zero. Thus, it is possible to achieve a very small overall TEL with a design that transmits very little (manifested by a low values of TR(λ)) at wavelengths where the emissivity is high (manifested by high values of I(λ)EL), and transmits substantially at wavelengths where the emissivity is low.
In other words, an EL display structure can have a relatively high photopic transmission (say, 60%), leading to a rather low photopic reflectance (100%−60%=40%, assuming small or negligible losses). Such a structure is substantially transparent to the sense of sight. However, at the same time, its electroluminescent transmission can be very low (in the order of 5%-35%), leading to a very high electroluminescent reflectance (65%-95%, again assuming zero or otherwise negligible losses). Such a structure is almost entirely non-transparent to electroluminescent light. The difference in photopic and electroluminescent values is created mostly by the different spectral characteristics of the light sources in TP and TEL and wavelength-specific response of the structure. If such a structure substantially reflects light on a narrow band of wavelengths, it is often called a narrowband reflector (“NBR” for short).
Depending on the application, values for suitable electroluminescent transmission of the narrowband reflector of the transparent thin film electroluminescent display of the present invention are from 50% to 65%, from 25% to 50%, or from 0.1% to 25% for the emission spectrum emitted by the active layer of the display. Assuming small losses (L is approximately 0), this leads to electroluminescent reflectance (REL=1−TEL) of from 35% to 50%, from 50% to 75%, or from 75% to 99.9% for the emission spectrum emitted by the active layer of the display, respectively.
In prior art, the problem of lack of contrast in bright ambient light has usually been solved with a specific backside photochromic layer that darkens the rear of the display element, as provided for example in US patent publication U.S. Pat. No. 5,757,127. Naturally, driving and control circuitry must be provided for the darkening layer for excitation and detection of bright ambient light, making such approach complex and requiring external energy. The approach is also based on absorption of light, not on the reflection of light. Thus, light emitted by the display towards the backside is lost from viewing purposes, and thus part of the energy driving the display is wasted.
Prior art also shows display devices where narrowband reflectors are used. For example WO2005/064383 uses a narrow band reflector for separating two stacked displays. This document does not present anything about the lack of display's contrast in bright ambient light as the overall display device is completely non-transparent and therefore not prone to ambient light passing through the overall structure.